If you want to improve the foundation of your skillset, I would highly suggest playing a lot of platter and rain diffs. I know people suggest converts but the patterns generated can vary greatly from map to map, which if you want a broader skillset you can feel free to go for converts but in my opinion, the CTB specific maps around rain and platter diffs are your best bet for improvement. I would also recommend trying out unranked CTB maps with at least 5 favourites as they can provide a good bit of enjoyment and improvement.